1. Field of the Invention
This invention relates to display systems using spatial light modulators, more particularly to the data handling for such systems.
2. Background Art
Spatial light modulators have many different forms. A common form has an array of individually addressable elements, each of which represent a picture element in an image being displayed. Two examples of spatial light modulators are the liquid crystal display devices (LCD) and the digital micromirror device (DMD, also known as the deformable mirror device).
The liquid crystal device typically functions as a transmissive modulator. The optical system is positioned such that the light passes through the LCD. The individual elements are activated and deactivated to block or transmit the light to the screen. They can also control the color. The DMD is a reflective modulator, with the optical system positioned to allow the individual elements to either reflect light to the screen or away from it. The individual elements typically receive a signal that causes the mirror to deflect in one direction or another. When it deflects in one direction, the light is reflected to the screen, when it deflects in the other direction, light is moved away from the screen.
Because of the ease of turning these elements, whether transmissive or reflective, ON and OFF, it is simple to operate them digitally using binary data. One problem with digital operation arises from a common form of pulse width modulation. In order to achieve varying levels of intensity (gray levels), in color or not, is to control the amount of time each level is on digitally. For example, for 16 levels of intensity, each element would have 4 bits of data. In binary weighting, the most significant bit (MSB) would be given 8/15 of the available time, such as a video frame time, to display its data. The next MSB would be given a 4/15, the next to least significant bit (LSB) would be give 2/15 and the LSB would receive 1/15.
The various combinations of these bits' on times including black, totals up to 16 levels of intensity. However, this manner of addressing can lead to visual artifacts in the image. For example, if in one frame, a pixel has an intensity level of 7, it would require the three lowest bits (bits 0, 1 and 2) to all be ON, and the MSB, (bit 3) to be OFF. If in the next frame, the level is 8, which is only one level away, all of the bits must change intensifies. The MSB would be ON, when it had been OFF before. The other 3 bits must then all turn OFF, when they had been ON. This point in the scheme, where every bit is changing state will be referred to as a bit transition. This causes visual artifacts in the image, taking away from the clarity and resolution of the image displayed.
Therefore, a method of preventing these artifacts while maintaining a good level of resolution is needed.